The invention concerns a slurry for cementing, especially cementing of lining pipes into drill holes. It also includes a method for production of such a slurry.
Some places it is necessary to tighten openings being subject to pressure from fluids with a cementing slurry, which should be mechanically stable after hardening. This is the case in oil drilling, etc.
In order to protect the integrity of drilled wells, a lining pipe is led down to the bottom. The annulum between the well wall and the lining pipe is filled with a cementing slurry, for instance a cement slurry, from the bottom and completely or partially up to the surface. The lining pipe should prevent solid particles and fluids (oil, gas, and water) from penetrating into the well. The cement slurry composition should primarily prevent communication on the external surface of the lining pipe. In hardened condition it should secure the lining pipe to the surroundings.
These objects are met with satisfaction if the cement slurry composition is placed completely around the lining pipe, and is allowed to harden or solidify without being interrupted. The cement permeability must be less than 0.1 mD (milliDarcy) and the compression strength higher than 100 psi.
Norwegian patent specification No. 144047 discloses a hardening mixture for cementing deep drill holes. A method for such cementing is known from U.S. Pat. No. 3,884,302. Neither of these techniques is totally satisfactory for the purpose.
In ground formations containing gas, known techniques will frequently create problems with the steaming of gas in the annulum space when the cementing slurry is added. Gas can often stream into minor channels or pores in the cementing slurry from a high pressure zone, and into a low pressure zone. Sometimes the gas can penetrate to the surface, or it can be stopped at the well head. This type of gas migration can be dangerous and give blowouts and accidents. Even if the leakage is discovered before this happens, expensive repairs will be necessary.
Several experiments have been made in the nature of these problems.
At the transition from liquid to hardened cementing slurry, a decrease in the hydrostatic pressure can give gas penetration.
Free water that is emitted during the hardening and settling process can develop water pockets in the hardened body. These water pockets can evolve into communication channels for gas when the water is suppressed.
Chemical processes, temperature variations, and filtrate loss can cause creeping of the cementing slurry during the solidifying and hardening period. Mechanical stress during the following drilling and perforating operations may create cracks in the cement.
Difficulties in obtaining good contact with the cementing slurry and the environment, and in suppressing the drilling mud may cause later creation of channels and gas migration. These difficulties have a close relationship to the filter cake of the drilling mud, and its properties. A thin and strong filter cake gives good mud suppression and causes no communication.
Before the solidfying process starts, the cement behaves like a liquid transferring hydrostatic pressure depending on density and depth. Early in the process, the cement stops behaving like a liquid, but rather as a plastic slurry with weak bonds and with free water in the voids between them. A reduction of the volume of the free water in the void structure of the cementing slurry will then cause a pressure reduction in the annulum. The volume can be reduced in two ways. In the solidifying period the water volume is reduced by 0.95-2% because parts of the water create chemical bonds with the cement. The other reduction occurs if some water can leak into the surrounding porous formation. Even with additional substances in the cementing slurry, which reduces the filtrate, this water loss cannot be totally prevented. When the pressure in the cementing slurry sinks under the formation pressure of the gas, it will easily penetrate into the cementing slurry. Gas bubbles will migrate upwards in the solidified slurry and leave channels through which more gas can stream.
Additives are suggested to maintain the pressure in the cement in the solidifying period. These additives create gas bubbles with equal pressure to that of the cementing slurry. By dispersion into the cementing slurry, the gas bubbles will expand when the pressure decreases and thereby maintain the pressure. It is then necessary to have good gas dispersion in order to prevent gas accumulations and channel formation, which is not always obtained.
The problem with the formation of water channels is created because water must be added to the cement slurry in order to enable pumping of the mixture for sufficient time to place the slurry in the well. The problem cannot be solved by letting some water escape by filter loss, because this will cause great pressure reductions in the cementing slurry, but a certain filter loss is desired in order to reduce the amount of free water. Methods for binding the free water that is generating during the solidifying are known, such as addition of bentonite (clay), pozzolan, expanding perlite etc. These additives have little or no effect at the pressure reduction, but give a certain negative effect on other properties of the cementing slurry, such as the ability to be pumped, and the strength.